This invention relates generally to filter changers for fluids comprising apparatus to replace a clogged filter with a fresh filter within a fluid flow passage. More particularly, it relates to filter changers useful in the polymer industry for the filtration of heated, pressurized polymer in liquid form as it flows from an extruder to any of various types of polymer forming equipment.
Typically, in the polymer application two or more filters are mounted in apertures on a carrier assembly adapted to locate the filters successively in an operative position that communicates between upstream and downstream polymer flow passages. The filter carrier assembly slides within a channel that extends transversely of the polymer flow passages through the operative position to the exterior of the filter changer, whereby clogged filters are made accessible for cleaning or replacement with fresh filters.
U.S. Pat. No. 4,237,014 to Trott and U.S. Pat. No. 3,675,934 to Heston illustrate typical known slide plate screen changers. These patents disclose two distinct sealing mechanisms intended to prevent leakage of polymer into the transducer channel. In the Trott patent sealing force is applied by means of a mechanically loaded assembly. Under predefined displacement the sealing surfaces are brought into contact with sufficient force to resist leakage at low operating polymer pressures. The Heston patent illustrates a different sealing mechanism whereby the polymer pressure is used to effect the sealing force. In either case the polymer or mechanical pressure is applied to a seal member, causing the seal member to bear on the upstream side of the filter carrier assembly or the filter then located in the operative position, thereby containing the polymer and preventing its flow into the filter channel. In this type of changer the sealing surface area is generally of small size and located close to the filter aperture, but it is sufficient to comply with, and remain in sealing contact with, small irregularities in the mating surfaces of the filter carrier, the filter channel and the flow passages.
Difficulties with the sealing means previously employed arise under conditions when the polymer pressure is substantially below the elevated level at which the filter changer is operated in normal use. Such conditions exist, for example, at start up or during idle periods of operation. During such periods the pressure exerted on the upstream seal is insufficient to prevent leakage of fluid into the transverse filter channel.
Problems also arise if filter changer assemblies cannot be easily disassembled for purposes of maintenance.
Accordingly, a principal object of this invention is to provide improved sealing means that are effective not only at normal operating pressures of the polymer, but during start up and idle periods when the polymer pressure is low.
A second object is to provide sealing means effective to establish a threshold force applied to the upstream seal that is effective to prevent polymer leakage when the polymer pressure is below a predetermined value.
A further object of the invention is to provide sealing means that are adapted to allow simple maintenance and disassembly procedures.